1. Field of Invention
The invention relates to a driving circuit, and in particular, to a lamp driving circuit.
2. Related Art
Recently, applications of flat panel displays have become increasingly popular. Among flat panel displays, the liquid crystal display (LCD) has become a mainstay of the market. With the development of liquid crystal display technology and in response to the requirements of the actual use of large scale displays, the number of lamps, e.g. cold cathode fluorescent lamps (CCFLs) serving as a backlight source, must be increased in order to provide the sufficient luminance. The prior art measures a feedback voltage of a transformer or a feedback current of the lamps to control the voltages to drive the lamps, thus ensuring the lamps' uniform light emission.
Referring to FIG. 1, a conventional lamp driving circuit 1 includes a power switching circuit 11, a transformer 12, a feedback circuit 13 and a power control circuit 14. The power switching circuit 11 receives a power PWR and generates an input current Iin. A primary side 121 of the transformer 12 is electrically connected with the power switching circuit 11 so as to transform the level of the input current Iin and to drive a plurality of lamps 2 on a secondary side 122 of the transformer 12. The current of the lamps 2 is fed back to the feedback circuit 13 which outputs a feedback voltage VFB. The power control circuit 14 controls a switching frequency of the power switching circuit 11 by way of pulse width modulation (PWM) according to the variation of the feedback voltage VFB in order to control the power switching circuit 11 to adjust the current for driving the lamps 2.
However, the feedback circuit 13 cannot individually process each current for driving the lamp 2, and can only receive the current corresponding to part of the lamps 2 as a feedback signal. Thus, the current difference between each of the lamps 2 will influence the current precision. In addition, when the lamps 2 cannot be grounded in response to the actual connection condition, the feedback circuit 13 cannot be directly connected to the lamps 2. Thus, the feedback control method of the driving circuit 1 has to be modified.
FIG. 2 shows another conventional lamp driving circuit 1A. In order to improve the drawbacks of the above-mentioned architecture, the primary side 121 of the transformer 12 may be serially connected to a primary side 151 of an induction transformer 15. A secondary side 152 of the induction transformer 15 generates the feedback voltage VFB, which is inputted to the power control circuit 14 to control the power switching circuit 11, to adjust the current for driving the lamps 2. In addition, the output power of the transformer 12 has to be increased so as to output a sufficiently high current for driving the lamps 2. At this time, the output power of the induction transformer 15 has to be increased with the enhancement of the transformer 12 so as to withstand large currents, and the size of the induction transformer 15 also has to be enlarged. If the voltage can be directly sampled, the product can be adapted to the non-grounding, non-feedback architecture of the lamp 2, yet without the inconvenient requirements of direct current measurement of the transformer 12 and high current capacity.
Thus, it is an important subject to provide a lamp driving circuit capable of preventing the above-mentioned problems from occurring and thus obviating the above-mentioned inconvenient requirements. Thus, the voltage across two ends of the transformer for driving the lamps can be directly measured, and the feedback control efficiency can be enhanced.